Conventional rotation shafts provided in various drive force transmission mechanisms are rotatably supported via bearings. For such rotation shafts, so-called ball bearings are used in order to improve rotation performance of the rotation shafts. Such ball bearings include multiple rolling members (rolling elements) circumferentially disposed between inner and outer rings.
Such bearings are used as support members for rotation shafts of drive force transmission mechanisms in various drive units, and some drive units are desired to prevent foreign substances such as dusts and water from penetrating through the bearings into the units. Moreover, when foreign substances enter into the bearings themselves, rotation performance of such shafts could be deteriorated and noise could be caused. To prevent this, an elastic seal member is provided on a portion of the periphery of a rotation shaft proximate to a bearing to shut out water and dusts from the bearing, however, the rotation performance of the rotation shaft may be degraded due to contact pressure caused by the elastic seal member. Especially for a double-bearing type fishing reel (a baitcasting reel) which also serves as a drive unit requires smooth rotation of a rotation shaft that rotatably supports a spool, and it is desirable that a bearing of the reel is formed in such a manner as to shut out foreign substances and have a low torque.
A magnetic fluid sealed bearing (a bearing with a magnetic sealing mechanism) is known for preventing foreign substances from penetrating through the bearing without degrading the rotation performance of the rotation shaft. For example, Japanese Patent Application Publication No. 2013-110 (the “'110 Publication”) discloses a fishing reel having a built-in bearing with a magnetic sealing mechanism wherein the bearing supporting a rotation shaft rotationally driven by handle operation is sealed with a magnetic fluid. The bearing of the fishing reel disclosed in the '110 Publication has a sealing structure wherein a magnetic fluid is retained between an inner ring or an outer ring of the bearing and a retaining plate (polar plate) retains a magnet. More specifically, magnetic body is disposed between the inner ring and the outer ring to block a rolling member, one side of the magnetic body is fixed, and a magnetic fluid is filled in a clearance on the other side of the magnetic body thereby the rolling member is hermetically sealed. In this way, penetration of foreign substances into the rolling member section can be prevented.
However, the bearing disclosed in the '110 Publication cannot securely shut out foreign substances entering from the fixed sides of the magnet and the polar plate. When this bearing is applied to a fishing reel used in a harsh environment where seawater, dusts and the like tend to adhere or penetrate, the seawater enters inside the reel, and the seawater in the bearing may degrade smoothness of the bearing, making it impossible to maintain the smooth rotation for a long period.
Especially when the bearing of the '110 Publication is provided in a baitcasting-type fishing reel (double-bearing reel), it is preferably configured to have a reduced torque in addition to the water and dust prevention feature. However, as closely studying components of the magnetic fluid sealed bearing, there are some factors that make it difficult to reduce the torque. One of the factors is a magnetic circuit formed by the magnet and the polar plate retaining the magnet when the magnetic fluid seal is disposed between the inner and outer rings of the bearing. Due to the magnetic circuit, a force to attract the inner and outer rings closer to each other is generated, and this force can be a load (resistance) to rotation of the rolling member. In this case, when the clearance between the polar plate and the inner ring (or outer ring) is small, the magnetic force of the formed magnetic circuit increases, which increases retaining capability (sealing effect) of the magnetic fluid. However, the attraction force to bring the inner ring and the outer ring close to each other is also increased and thereby the load on the rolling member (rolling element) is increased, resulting in a high torque. Furthermore, when the magnetic force is generated on the rolling member side, the attraction force between the rolling member and the inner and outer rings with which the rolling member is contacted is increased, resulting in another load to increase the torque. In other words, in order to reduce the torque while enhancing the water and dust prevention feature in the configuration where the magnetic fluid seal is provided between the inner ring and the outer ring, it is important to adjust the magnetic force (generated by the magnetic circuit) that brings the inner and outer rings close to each other such that the seal effect is not lost and a strong magnetic field is not generated on the rolling member side.
The present disclosure is intended to overcome the above problems. An object of the present disclosure is to provide a magnetic fluid sealed bearing with which torque can be reduced without impairing a water and dust proof feature, and a fishing reel in which the magnetic fluid sealed bearing is provided.